The present invention generally relates to a process for preparing S-alkylthiopropionic acids, as well as derivatives thereof and the products thus produced. More particularly, the invention relates to the preparation of 3-alkylthiopropionic acids and derivatives by directly reacting a mercaptan with acrylic acid or acrylate salt, the direct reaction being an addition reaction carried out in the presence of a strong base catalyst. When the 3-alkylthiopropionic acid is esterified with pentaerythritol or the like, the product formed has an extremely high level of ester having the tetraester structure.
Alkyl esters derived from alkylthioalkanoic acids and the like are, in general, known to be useful as stabilizers of organic materials such as polymer resins and the like which are otherwise subject to thermal and oxidative deterioration during processing, extrusion or molding, as well as during use. Esters having this general utility have in the past been prepared by various procedures. Dexter et al U.S. Pat. No. 3,758,549, for example, basically teaches transesterification procedures for the preparation of these types of products. By such procedures, it is often difficult to obtain a product that has a tetraester content at or above 90% by weight, particularly when the transesterification is carried out on an industrial scale.
Stabilizers for enhancing the resistance of polyolefins to deterioration can also be prepared by reacting an alpha-olefin with a multi-functional ester of a mercaptocarboxylic acid. Stabilizers of this type and the process for their preparation are disclosed in Kauder et al U.S. Pat. No. 4,080,364. Experience with this type of addition reaction indicates the product thus formed has a tetraester content which typically does not meet or exceed 90% by weight.
Nakahara et al U.S. Pat. No. 4,349,468 teaches the preparation of a pentaerythritol tetrakis (3-laurylthiopropionate) stabilizer for polyolefins which is produced by a process including heating an alpha-olefin such as 1-dodecene with a beta-mercaptopropionic acid or ester in the presence of an azonitrile or peroxide catalyst, followed by esterifying the resultant alkylthiopropionic acid with pentaerythritol. The resulting product is typically inferior in that the alpha-olefin reaction produces an unwanted isomer byproduct that, if not removed in a separate purification step, lowers the quality of the pentaerythritol ester.
Alkylthiopropionic acids prepared by approaches such as these do not typically directly produce, without special purification, an alkylthiopropionic acid which exhibits a high purity characteristic which will, when reacted with pentaerythritol or the like, form an ester product which has an extremely high tetraester content and a minimum of other components such as the triester. Furthermore, this advantageous result is typically achieved by the present invention without requiring a substantially high excess of the acid reagent, thereby minimizing the cost and inefficiency of having to remove excess acid from the reaction product and without purification at an intermediate stage.
By the present invention, a 3-alkylthiopropionic acid having about 4 to about 20 carbon atoms in the alkyl group is prepared by directly reacting an alkyl mercaptan having between about 4 and about 20 carbon atoms with an alkali metal acrylate, the reaction being carried out in the presence of a strong base catalyst, typically within an aqueous alkaline solution having a pH of at least about 11. Thereafter, the reaction solution is acidified to a pH which is at about 4 or below, and the 3-alkylthiopropionic acid is then recovered from the water-insoluble phase of the acidified reaction solution. When desired, a high purity tetraester product is formed by esterification of the 3-alkylthiopropionic acid with pentaerythritol or the like.
It is accordingly a general object of the present invention to provide an improved process for preparing an alkylthiopropionic acid and to provide an improved acid prepared by the process.
Another object of the present invention is to provide an improved process for preparing an S-alkylthiopropionic acid by a direct addition reaction process which results in the recovery of an S-alkylthiopropionic acid which is of exceptional purity.
Another object of this invention is to provide an improved method for providing a 3-alkylthiopropionic acid which is especially suitable for use in preparing a 3-alkylthiopropionate tetraester.
Another object of the present invention is to provide an improved process for the work-up purification of a tetraester formed from the 3-alkylthiopropionic acid.
These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description.